With the recent explosive increase in demand for radio communications, the variety of radio communication methods as well as the number of radio communication enterprises have increased. In addition, broadening the frequency band for a wireless communication systems is being promoted correspondingly, whereby the frequency band available for radio communication dries up.
Considering this trend, recently a solution in which plural systems share the same frequency band has been proposed and reduced to practice. For example, there is a frequency band as referred to as an ISM (Industrial Scientific Medical) band. As for this frequency band, a radio license is unnecessary, and many radio communication enterprises such as wireless LAN (Local Area Network) enterprises use this band to provide their communication services. There is a possibility that sharing of frequency bands between the plural systems will find increasing use from now on.
FIG. 1 is a diagram showing an example of a method of sharing frequency bands between plural systems. Referring to FIG. 1, according to example 1, the plural systems are defined as different wireless LAN enterprises “provider A” and “provider B” which use the same wireless interface “IEEE.11a”. Similarly, according to example 2, the plural systems are defined as two enterprises “operator A” and “operator B” which use the same wireless interface “W-CDMA” (Wideband Code Division Multiple Access). According to example 3, the plural systems are defined as different wireless interfaces “W-CDMA” and “PHS” (Personal Handyphone System) within the same enterprise “operator A”. According to example 4, the plural systems are defined as different enterprises “operator A” and “provider B” which also use different wireless interfaces “W-CDMA” and “IEEE.11a”, respectively. In this way, various definitions can be considered as plural systems.
By the way, a problem that occurs when the plural systems share the same frequency is interference between systems. In order to assure a certain level of communications quality in each system, a technique for preventing interference (interference cancellation technique) becomes necessary.
Here, with reference to FIG. 2, an interference cancellation technique used in existing wireless LAN systems is explained as an example.
FIG. 2 is a diagram showing a conventional configuration of transmitters and receivers under the circumstance where plural systems coexist, in which systems 1, 2 . . . N indicate wireless communication systems provided by different wireless LAN enterprises, respectively. Each system is provided with a transmitter 101 including a signal generating section 102 and a timeslot allocating section 103, and a receiver 105 including an antenna 106 and a decoder 107 and opposed to the transmitter 101. It is noted that the transmitters 101 of the respective systems 1, 2 . . . N are connected to a wired network NW such as the Internet.
Although these systems 1, 2 . . . N use the same frequency band, interference between systems is prevented as follows; each transmitter 101 detects that the transmitters 101 of other systems are not in operation for transmission, by using protocols such as CSMA-CA (Carrier Sense Multiple Access-Collision Avoidance). The transmitter 101 transmits data to the corresponding receiver 105, if other transmitters 101 are not in operation for transmission. In this way, it is possible to implement time division of communications between systems and thus prevent interference between systems.
However, according to the aforementioned prior art in which interference between systems is prevented by using protocols such as CSMA-CA, the communications between systems is performed according to a time division scheme, so there is a problem in that frequency use efficiency per system decreases 1/N times with respect to the case of only one system at a time allocated for a frequency band. FIG. 3 is a diagram showing a frequency use situation by each system on a time axis. It is shown that only one system can utilize the frequency band at the same time.
Therefore, there is a need for a technique which improves the frequency use efficiency, since it is likely that the number of the radio communication enterprises (=the number of systems N) will increase further.